Peter van Mourik

152 Chapter 7 modulator responses in subjects where clinical domain indicators are unsuited to measure therapeutic response. There are several limitations in this study. First, the open-label setting of treatments can induce bias in the acquisition of clinical data. Potentially ppFEV 1 might have been influenced, but this is unlikely for SCC measurements. However, we do not expect that the open label setting has strongly affected the in vitro-in vivo correlation, since the clinical observers and patients were blinded for the in vitro drug responses and vice versa. Second, the study is biased for potentiator treatments. The area under the ROC-curves may be different when patients are stratified for different CFTR modulator treatments such as corrector/potentiator combinations. Also the cut-off values of ppFEV 1 and SCC that were used to define a clinical responder may not be fully accurate in identifying long-term clinical responders to treatment, and changing these cut-off values will lead to different ROC-curves. Third, patient subgroups with differences in organoid baseline CFTR functions may require different organoid test conditions (e.g. different forskolin conditions) for better predictive values. Fourth, it remains challenging to estimate adequate drug concentrations in the organoid tests as to optimally reflect the in vivo tissue concentration. For ivacaftor and lumacaftor we relied on average blood concentrations to determine the in vitro drug concentrations 38,39 . For genistein and curcumin, lack of information on in vivo tissue concentrations may resulted in overdosing the in vitro situation , which can lead to overestimation of their potential in vivo effect. Most importantly, larger follow up studies remain needed to define more precisely how organoid-based measurements, and possibly other short term endpoints, can predict long term individual benefit to various CFTR modulator treatments. Apart from the performance of FIS as a biomarker of treatment response in this study, the rectal organoids provide additional benefits over other biomarkers of CFTR function. Rectal organoids are adult stem cell cultures that can be generated from a single rectal biopsy and cultured over 6 months while maintaining patient-specific CFTR modulator response 16,40 . Rectal biopsies are accessible in most subjects independent of age and can be shipped to dedicated centers for organoid testing within weeks and stored in living biobanks which enables future drug testing 16 . The FIS readout appears also not affected by CF disease phenotype (e.g. irreversible damage and inflammation in pulmonary markers). Currently, the immediate impact can be the selection of people for treatments independent of the CFTR genotype, both for CFTR modulators on the market and in development. For people having access to treatment, we may be able to further individually tailor treatments to maximize clinical benefits 41 .